The introduced seaweed Caulerpa taxifolia is an extremely invasive saltwater alga that has been discovered in South Australia. The noxious weed was discovered in West Lakes in metropolitan Adelaide in 2002 and was successfully killed using fresh water. The weed was also discovered in the Port River at the same time as the West Lakes infestation and has since spread down river to near the old Quarantine Station, around Garden Island and into Eastern Passage, North Arm and the southern end of Barker Inlet. This is despite concerted eradication efforts using salt, chlorine and suction. There are several reasons for this spread; the 2004-2005 long, hot summer and mild start to winter provided ideal conditions for the weed to thrive. Caulerpa taxifolia’s ideal habitat is warm water. The State Government is moving swiftly on plans to contain and control the weed to prevent it from spreading further – and certainly from spreading into Gulf St Vincent.

What is Caulerpa taxifolia?

Caulerpa taxifolia is a bright green seaweed with main stems that can grow up to three metres long, with up to 200 fronds growing off these stems that look like feathers or ferns. The weed can live as individual plants or grow to form dense blankets covering large areas of riverbed. It can grow up to 25mm a day and spreads by itself. New infestations can start from tiny fragments. It’s a tropical species and some strains are native to the warm waters of northern Australia – but a strain capable of surviving and thriving in nontropical waters is invading the Port River. It has been nominated as one of the world’s top 100 worst invasive species and probably came to South Australia from an aquarium.

 Why is it bad news for SA?

The invasive weed spreads quickly over large areas of the seafloor and takes over areas occupied by native species such as sea grasses. It is extremely difficult to eradicate. It has the potential (based on its known depth and temperature ranges, and invasive history) to spread over most South Australian waters including Gulf St Vincent and Spencer Gulf, which means it is a threat to seagrass meadows and biodiversity. If a piece of Caulerpa taxifolia was accidentally kept in a warm, damp environment such as an anchor well, or on fishing or diving gear and later returned to the water, it could grow to establish a new colony in a new area. Where it invades and pushes out native flora, it also destroys habitats for fish, especially nursery habitats, meaning fish can no longer find feeding and breeding grounds. Fish do not generally eat Caulerpa taxifolia because it contains a toxin that makes it distasteful to them. In other parts of the world it has already had a devastating impact on marine environments, out-competing indigenous seaweeds and displacing bottom-dwelling marine communities. This means it threatens our $200 million wild fishing industry and the State’s most popular pastime – going fishing.

What has been done?

There have been large-scale efforts to remove the weed since it was first discovered in West Lakes in March 2002. When it was subsequently found in upper reaches of the Port River near Jervois Bridge, the State Government undertook intensive eradication campaigns. These targeted treatments will continue on weed patches. West Lakes was sealed off and filled with freshwater from the River Torrens. This worked extremely well and, as of May 2005, the lakes remain clear of the weed. But because the river could not be sealed off, other measures had to be taken. These included removal of patches of the weed by divers using suction pumps, smothering by salt or black plastic sheeting, with chlorine pumped under the plastic in high-risk areas. The suction treatment was more than 95% effective while the salt treatment, which involved, 5800 tonnes of salt over areas totalling 60,000 square metres was 97% effective. The black plastic treatment was 100% effective, but could not be delivered to large areas.

What is still being done?

PIRSA and SARDI Aquatic Sciences are continuing to survey and monitor areas of the Port River, North Haven and Barker Inlet to detect infestations and new colonies of Caulerpa taxifolia. Fringe control, with divers tackling and containing patches with suction pumps, is taking place in Barker Inlet and in the Port River near the old quarantine station. This targeted control of infested areas also covers areas where there is known to be a higher risk, such as near boat moorings, around the Torrens Island power station warm water outlet, and where tides and currents might deposit it. PIRSA is also creating risk mitigation plans for construction and dredging sites, such as near the Port redevelopment and the third River crossing. Finally, a major public education campaign is underway to inform everyone using the waterway of the problem and how they can help.

What can you do?

DON’T THROW THE WEED BACK INTO THE WATER

• LEARN to recognise Caulerpa taxifolia. It is important for commercial and recreational water users to notice what species are normally present in the marine habitat so they can recognise anything unusual or species showing an abnormal change in abundance. Please check PIRSA’s pamphlets, posters and website.
• AVOID disturbing it with boat anchors or fishing tackle.
• CHECK anchors, trailers, ropes, and chains before you set out and when you come ashore.
• CHECK fishing tackle – lines, hooks, floats and sinkers – every time.
• CHECK all diving and boating equipment including wetsuits, bags, towropes and other gear such as paddles.
• COLLECT and remove any Caulerpa taxifolia you find on your equipment and dispose of it in a sealed, biodegradable bag.
• WASH ropes, tackle and other equipment in a plastic bin using freshwater and household bleach.
• DO NOT use the plant in an aquarium or release any water, plants or animals from a saltwater aquarium into drain, creek, river or sea.
• OBSERVE any restrictions.

People who see the weed should note where and when they saw it, water depth (if possible), type of siting (e.g. while diving, on an anchor chain, from the shore), whether it was isolated or a big patch, and give PIRSA your contact details by calling the 1800 065 522 hotline.